#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main()
{
    ll n;
    cin >> n;
    ll ans = 0;
    ll cnt = 0, x = 1;
    vector<ll> a(n + 1);
    for (ll i = 1; i <= n; i++)
    {
        cin >> a[i];
        ans = ans + x * a[i];
        cnt++;
        if (cnt == 2)
        {
            cnt = 0;
            x *= -1;
        }
    }
    cout << ans << endl;
}